Common rail fuel systems have shown considerable promise in providing the versatility necessary to improve performance while also reducing undesirable emissions, especially in relation to compression ignition engines. As the industry demands ever higher injection pressures, more problems have begun to reveal themselves. Among these problems may be a need to cool an internal electrical actuator, such as a solenoid or piezo, in order to maintain the electrical actuator in a temperature range that maintains high actuation forces coupled with fast response times. In some applications, especially those having electrical actuator spatial constraints, maintaining and improving actuator performance can be problematic. For instance, in many applications, one or more electrical actuators must be totally contained within an injector body, and a certain proportion of the electrical actuator, especially in the case of solenoids, must normally be occupied by insulating material. Thus, in the case of solenoid actuators, maintaining or improving flux transfer while also reducing the volume of material associated with insulating properties can be problematic. Prior art solenoid assemblies for fuel injectors typically include a pole piece upon which is mounted a plastic bobbin that carries the solenoid coil winding. Because the winding is typically wound onto the bobbin before attachment to the pole piece, the bobbin must have sufficient structural integrity to undergo the winding process. The end result might be more material volume being associated with the bobbin than might otherwise be needed for proper operation after the solenoid is installed.
In a typical fuel injector application, a solenoid actuator is coupled to a valve member to open and close one or more fluid passages to facilitate a fuel injection event. Two types of solenoids have appeared in the art. One type is identified as a dual pole solenoid and often is characterized by the fact that the peripheral edges of the armature have a diameter larger than the outer diameter of the coil winding. The armature moves between an initial air gap position and a final axial air gap position with regard to a stator. In another type, a so called single pole solenoid includes not only an axial air gap but a sliding air gap within which the armature moves. One such example is shown, for instance, in Coltec Industries Inc.'s U.S. Pat. No. 4,984,549 to Mesenich. Single pole solenoids are often identified by their armature peripheral edge having a sliding flux gap with a magnetic flux carrying member, and the diameter of the armature is typically smaller than the inner diameter of the coil winding. Regardless of the solenoid type, the flux transfer capability of the solenoid assembly, and hence the speed and responsiveness of the associated valve, can deteriorate substantially as temperatures increase beyond a certain level depending upon the solenoid structure and materials used. Increased temperatures can be attributed to leakage within the fuel injector, repeated actuation events, and even the transfer of temperature from the combustion chamber of the engine through other fuel injector components.
Another important feature that affects the performance of solenoids relates to the size of air gaps that separate the moving armature from stationary magnetic flux carrying components of the solenoid assembly. While smaller air gaps may facilitate better flux transfer, geometrical variations in component parts may make mass production of solenoid assemblies with uniform air gaps that yield consistent behavior illusive. For instance, maintaining smaller air gaps often requires the armature to be guided in its motion, such as via attachment to a valve member which moves in a guide clearance bore. However, geometrical tolerance stack-ups may limit the realistic air gaps available with such a strategy.
The present disclosure is directed toward one or more of the problems set forth above.